Nonequilibrium steady-state differences in partial pressure of CO 2 and in concentration of weak acids and bases between blood and tissue.

Several investigators have demonstrated that under conditions where little or no gas exchange occurs across the alveolar capillary membrane the PCO(2) is higher in the alveolus than in the mixed venous blood, whereas there are no PO(2) differences. Gurtner et al. have explained the DeltaPCO(2) by a model in which H(+) dissociation of proteins due to an electrical field caused by a negatively charged capillary wall (Wien effect) sets up an intracapillary PCO(2) difference between wall and bulk phase which is maintained by blood flow. The model is not specific for CO(2) and predicts that weak acids should be concentrated in a manner similar to CO(2) whereas weak bases should be relatively excluded from the alveolar space. Measurements of the steady-state distribution of the uncharged forms of the weak acids 5,5-dimethyloxyazoladinedione (DMO) and barbital and of the weak base tris(hydroxymethyl)aminomethane (THAM) between mixed venous blood and a fluid-filled lobe of lung were made in living dogs. The results agree fairly well with the predicted values.